How to Fertilize Perennials
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How to fertilize perennials starts with a question most gardeners skip: does the bed need fertilizer at all? The UMass Extension, University of Maryland Extension and Illinois Extension pages we opened agree that most established perennials need little feeding, and that a soil test or a yearly layer of compost should set the amount. They differ on timing and on how much nitrogen to apply, so this guide shows where each source stands.
Quick answer
- Test the soil or add compost first, and fertilize only if the plants need it.
- Feed established perennials in spring, when growth resumes, with a complete granular fertilizer.
- Aim for about one pound of nitrogen per 1,000 square feet, unless a soil test says otherwise.
- Water the bed after you apply fertilizer, and avoid overfeeding, which the sources link to weak, floppy growth.
Start with compost and a soil test
The gentlest feeding plan is to build the soil first. The University of Maryland page says that “If you add organic matter (compost, mulches, shredded leaves) on a regular basis, your plants may be adequately nourished” and may need little or no fertilizer. For perennial beds in particular, it recommends a spring routine: “Rake away mulch in spring and apply 1-inch of compost around plants.” The page puts that compost layer in its spring steps.
A soil test is the second step, and it decides the rest. The UMD page says “With less fertile soils, add fertilizer according to recommendations given by soil test results.” The UMass page describes the same reliance on testing, and says that the lime and fertilizer recommendations from its Soil and Plant Nutrient Testing Laboratory “are provided for a 100 square foot area.” Illinois takes a similar line, saying perennials can benefit from a set amount “unless a soil test indicates otherwise”.
Do most perennials need fertilizer?
Often, they do not. The Illinois Extension page says “Most perennials do not require large amounts of fertilizer if the soils have been prepared properly.” The UMass page makes a related point about the whole flower garden. It says that “Plant health, types of fertilizers, weather conditions, soil types and other factors influence plant nutrient needs and timing of application.” That list is a reminder that the bed, not the calendar, should set the schedule.
The sources also warn that too much feeding does harm. Illinois says “Many overfertilized perennials will produce excessive, soft growth and produce very few flowers.” It adds that overfertilized perennials may “lodge” or open up. A plant that flops over or grows lush leaves without flowers is a sign to stop feeding and check the soil before adding more.
Read the numbers on the bag
The three numbers on a fertilizer bag describe the nutrients in it. The UMass page explains that “The numbers on a fertilizer bag refer to the percentage of nitrogen (N), phosphorus (P 2 O 5 ) and potassium (K 2 O) (in this order) which are the nutrients needed in largest amounts by plants.” A 10-10-10 bag is 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium by weight.
For flowering plants, UMass says that complete fertilizers “often contain amounts of nitrogen less than or equal to the amounts of phosphorus (i.e. 10-10-10 or 5-10-5).” The reason it gives is that “phosphorus encourages flowering and excessive nitrogen will stimulate green leafy growth at the expense of flower production.” The Illinois page names the same kinds of product: “Granular fertilizers with a formulation of 12-12-12, 10-10-10, 5-10-5 or similar are sufficient.”
Phosphorus and potassium need a reason before they go on. UMass says that “However, phosphorus (P 2 O 5 ) and potassium (K 2 O) should be applied only if their need is indicated by a soil test.” It also warns that “Phosphorus can be moved in runoff water and poses a threat to water quality.” A soil test is the way to decide whether the bed needs either nutrient.
Work out the amount for your bed
The sources give rates in two forms, so it helps to convert them. Illinois says that, “As a general rule, unless a soil test indicates otherwise, perennials can benefit from one pound of nitrogen per 1000 sq. ft.” The UMD page gives a rate for the nitrogen per 100 square feet: “Apply approximately 0.1 lb. of nitrogen (N) per 100 sq. ft. of garden bed.” Those two figures agree, because one pound per 1,000 square feet is the same as 0.1 pound per 100 square feet.
The UMass page shows how to turn a nitrogen target into a bag amount. Its method is to divide the nitrogen the soil test recommends by the nitrogen percentage on the bag. The UMass example reads: “0.25 (lbs. of nitrogen based on recommendations) divided by 0.10 (10% nitrogen in fertilizer) = 2.5 lbs. of 10-10-10 fertilizer to supply 0.25 lb. nitrogen per 100 sq. ft.” The table below works the same formula for the two rates in these sources, using 10-10-10 as the example bag.
| Nitrogen target | Area | Calculation with 10-10-10 (10 percent nitrogen) | Pounds of 10-10-10 |
|---|---|---|---|
| 0.25 lb nitrogen (UMass soil test example) | 100 sq. ft. | 0.25 ÷ 0.10 | 2.5 |
| 0.1 lb nitrogen (UMD guidance) | 100 sq. ft. | 0.1 ÷ 0.10 | 1.0 |
| 1 lb nitrogen (Illinois general rule) | 1,000 sq. ft. | 1 ÷ 0.10 | 10 |
The table is arithmetic on the figures the sources give, not a result they report. Each row divides the nitrogen target by the nitrogen share in the bag. The UMass page says to divide the lbs. of material in the soil test by the percent of that same material in the fertilizer, so a bag with a different nitrogen share needs a different amount. Check the bag’s first number before you measure.
Feed in spring when growth resumes
Spring is the main window in these sources. The UMass page gives a short instruction for established plantings: “Apply fertilizer when growth resumes in the spring.” The UMD page gives the reason: “Apply fertilizer in the spring so it will not leach out before plants can benefit from it.”
Later in the season, UMD allows a second, lighter step. It says “Lightly fertilize plants if needed with a complete granular fertilizer” and that this “can be repeated mid-summer if growth is weak or foliage is light green in color.” For plants that bloom late in the year, it adds that “In late summer, fertilize plants that will bloom late summer through early fall.” The rule that runs through all these notes is to feed only when the plant shows a need.
Choose granular or slow-release products
Most home gardeners use a granular product, and the sources describe how long each type lasts. The UMass page says “Granular fertilizer formulations that are not controlled-release will generally supply nutrients to the plants for about 6 to 8 weeks.” Controlled-release products last longer. The page explains that “Controlled-release granular fertilizers consist of water soluble fertilizer that is encased in a semi-permeable resin coating”, and that “The rate of nutrient release for most of these fertilizers is regulated by temperature.”
Rain and watering also matter. UMass warns that “During periods of excessive rainfall or frequent irrigation, the nutrients may be leached out of the soil and fertilizer may need to be reapplied.” It adds that liquid feed has a use of its own, since “In the flower garden liquid fertilizer is useful for a quick boost or to supplement granular or controlled-release fertilizers when then they have been depleted.” A liquid feed is a supplement, not a replacement for the spring granules.
Put it on the soil and water it in
Placement is the next step. UMass says “If possible, pull back the mulch around plants so the fertilizer is applied to the soil and not on top of the mulch.” Pulling back mulch lets the granules reach the soil where the roots are. The UMD page says to “Water the bed after applying fertilizer”, and explains why: “This washes the fertilizer off the foliage, preventing fertilizer leaf burn, and makes nutrients available to the plants quickly.”
Mulch can change how much nitrogen the soil has. The UMD page warns that “Your garden may need additional nitrogen if you use straw, raw sawdust, or wood chip mulches because microorganisms decompose the mulch, taking up available nitrogen in the process.” A bed with a heavy wood-chip layer may need a little more nitrogen than the rate above, and a soil test can show whether it does.
Common mistakes to avoid
- Feeding too much. Illinois says overfertilized perennials “will produce excessive, soft growth and produce very few flowers.”
- Feeding phosphorus or potassium without a test. UMass says these “should be applied only if their need is indicated by a soil test.”
- Feeding before growth starts. UMass says to “Apply fertilizer when growth resumes in the spring.”
- Leaving granules on the mulch. UMass says to pull the mulch back so the fertilizer reaches the soil.
- Skipping the water. UMD says to water the bed after applying fertilizer, to wash it off the foliage and to make nutrients available.
How we chose
We began with the search question, how to fertilize perennials, and looked for university or extension pages that cover fertilizing perennial beds. We considered several pages and kept three that address the topic directly: the UMass Extension fact sheet “Fertilizing Flower Garden Plants”, the University of Maryland Extension resource “Care of Annuals and Perennials”, and the Illinois Extension “Planting and Care” page for flowers. We opened each page with curl on 2026-10-23 and confirmed that its text covers perennial feeding. Commercial gardening sites appeared in the results, and we did not use them for any rule or figure. We kept a rule only when one of the three pages states it, and we checked each quoted phrase against the text we downloaded.
What we could not verify
The pages do not agree on one rate, and this guide reports each figure with its source. None of the three pages is a perennial-only bulletin, and we did not confirm a publication date for each one, so check the rates against a current extension publication. The sources also do not say how much fertilizer to use for a bed in a specific region or soil type, beyond the general rules above. This guide makes no product claim and names no product.
What the sources do not say
- The sources do not give one nitrogen rate. Illinois gives one pound per 1,000 square feet, while UMD gives 0.1 pound per 100 square feet, which is the same amount on a different base.
- The sources do not give a fixed date for the last feeding. UMD allows a late-summer feed for plants that bloom into early fall, but it does not give a cutoff date.
- The sources do not describe how to fertilize plants grown in containers, since the pages are written for garden beds.
- The UMass page is written for flower gardens in general, so its examples include annuals and bulbs as well as perennials.
- The Illinois page is a planting and care page, so its feeding advice is short and general.
Frequently asked questions
Do perennials need fertilizer every year?
Not always. Illinois says most perennials do not require large amounts of fertilizer if the soil was prepared properly. UMD says regular organic matter may mean the plants are adequately nourished, and it recommends one inch of compost around plants in spring.
When should perennials be fertilized?
UMass says to apply fertilizer when growth resumes in the spring. UMD says to apply it in spring so it will not leach out before the plants can use it, and it allows a light repeat in midsummer if growth is weak.
How much fertilizer should be used?
The sources give a nitrogen rate, not a bag amount. Illinois suggests about one pound of nitrogen per 1,000 square feet unless a soil test says otherwise. UMD gives 0.1 pound per 100 square feet. Divide the nitrogen target by the nitrogen share on your bag to get the amount, as the table above shows.
Can compost replace fertilizer?
UMD says regular organic matter may mean the plants are adequately nourished, and it recommends applying one inch of compost around plants in spring. A soil test is still the best guide when the soil is poor, since UMD says less fertile soils need fertilizer according to the test results.
What happens when perennials are overfed?
Illinois says overfertilized perennials produce excessive, soft growth and very few flowers, and that they may lodge or open up. If you see that, stop feeding and check the soil before you apply more. For a related bed project, see the guide to soil for raised planters, and for fall preparation, see the guide to preparing raised beds for winter.
Sources
- https://www.umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/fertilizing-flower-garden-plants (opened 2026-10-23)
- http://extension.umd.edu/resource/care-annuals-and-perennials (opened 2026-10-23)
- https://extension.illinois.edu/flowers/planting-and-care (opened 2026-10-23)
